Production of charmonium in 250 GeV μ+-iron interactions

The production of J/ ϑ and ϑ′ has been measured in 250 GeV muon iron interactions. The measured total cross sections are σ( μN → μJ/ ϑX)=0.74±0.14 nb and σ( μN → μϑ′X)=0.16 ± 0.07 nb. An upper limit on the cross section times branching ratio for ϒ production of BR · σ ( μN → μϒX) < 5.2 × 10 −38 cm 2 (at 90% confidence level) is obtained. About half the J/ ϑ cross section is found to have Z ⩾ 0.95 (where Z= E(J/ ϑ/ ν). The first-order photon-gluon fusion model agrees well with the measured Q 2 and ν dependence of the J/ ϑ data and is used to extract the gluon momentum distribution. However, higher order QCD effects are needed to explain the Z distribution of the J/ ϑ and the observed broadening of the P t 2 distribution with decreasing Z. The decay angular distributions of the J/ ϑ are found to be flat in the s-channel frame, but there is evidence for polarisation in the t-channel frame.